Fix checking collaborators when publishing to external registry

[fohletex]

I experienced issues when working with np and referencing an external registry. Although I added

"publishConfig": {
    "registry": "https://fooregistry.tld"
 },

to my package.json file and having an additional reference in the local .npmrc-file, the check prerequisities command required a long time and finally failed. As #339 states an npm update might help, but my version was already 6.13.1.

The reason for my issue was that the npm.collaborators-command missed an additional arg option "--registry https://fooregistry.tld": This PR adds the required option and thus fixes the npm.collaborators-command in order to allow collaborator checks for external registries.

Although these issues are already closed, the error I experienced with the most-current np version comes closest to those:

• Fixes Command failed: npm access ls-collaborators X #339 (is closed, but behaviour was the same due to another reason)
• Fixes Prerequisite check runs forever #176 (is closed, but isn't clearly stated as fixed)

[itaisteinherz]

Thanks for the PR @fohletex!

[fohletex]

You are welcome! Thanks for merging. :)